The fabrication of large area thick film dielectric electroluminescent displays, for example, for television applications, requires substrates that can be fabricated in large areas, while still maintaining their dimensional stability during the heat treatment steps of the fabrication process. In particular, lower cost substrates exhibiting such properties are preferred, in order to lower the manufacturing costs of these displays.
In thick film dielectric electroluminescent displays, a display pixel is addressed by applying a voltage between a selected address row and a selected address column on opposite sides of a phosphor layer, which is sandwiched between two dielectric layers, one of which is a thick film dielectric layer. The applied voltage creates an electric field across the phosphor film at the pixel, located at the intersection of the selected row and column site. When the voltage across the pixel exceeds a threshold voltage, electrons from the interface between the phosphor layer and an adjacent dielectric layer are injected into the phosphor layer, rendering it electrically conductive and causing the entire applied voltage across the dielectric layers.
A thick film dielectric electroluminescent display is typically constructed on a glass, glass ceramic, ceramic, other heat resistant substrate or the like. The fabrication process for the display entails first depositing a set of lower electrodes on the substrate. A thick film dielectric layer is deposited next using thick film deposition techniques that are exemplified in U.S. Pat. No. 5,432,015 (the disclosure of which is incorporated herein by reference in its entirety). A thin film structure comprised of one or more thin film dielectric layers sandwiching one or more thin phosphor films is then deposited, followed by a set of optically transparent upper electrodes using vacuum techniques as exemplified by International Patent Application WO 00/70917 (the disclosure of which is incorporated herein in its entirety). The entire resulting structure is covered with a sealing layer that protects the thick and thin film structures from degradation due to moisture or other atmospheric contaminants.
A significant advantage of electroluminescent displays with thick film dielectric layers over traditional thin film electroluminescent (TFEL) displays is that the thick film high dielectric constant layer may be made sufficiently thick to prevent dielectric breakdown without a significant increase in the display operating voltage. The high relative dielectric constant of the materials that are used minimizes the voltage drop across the dielectric layer when a pixel is illuminated. In order to prevent dielectric breakdown, the thick film dielectric layer is typically comprised of a sintered perovskite, piezoelectric or ferroelectric material e.g. lead magnesium titanate-zirconate (PMN-PT) or lead magnesium niobate (PMN), with a relative dielectric constant of several thousand and a thickness greater than about 10 micrometers. An additional thinner overlayer of a compatible piezoelectric material or ferroelectric material e.g. lead zirconate titanate (PZT), may be applied using metal organic deposition (MOD) or sol-gel techniques, to smooth the surface of the thick film for subsequent deposition of a thin film phosphor structure.
When a thick film dielectric electroluminescent display is constructed on an alumina substrate, typically, thin film gold electrodes are applied to the alumina substrate and the thick film dielectric layer is deposited thereon. The thick film dielectric layer is sintered at about 850° C. to achieve a sintered thick film density that is sufficiently high that the remaining pores, particularly in the upper portion of the layer, may be filled by deposition of the thinner overlayer deposited using sol-gel or MOD techniques. The thinner overlayer, however, does not completely fill the pores of the sintered material since it undergoes a severe volume reduction when the sol-gel or MOD precursor materials are fired to form the piezoelectric or ferroelectric material of the thinner overlayer. To overcome this disadvantage, an isostatic pressing process is used to deposit the thick film dielectric layer and mechanically compress it before it is sintered as described in U.S. patent application Ser. No. 09/540,288 filed Mar. 31, 2000 (the entirety of which is incorporated herein by reference). This serves the function of increasing the density and decreasing the porosity of the thick film dielectric layer so that, when the thinner overlayer is applied, both the relative dielectric constant and the dielectric strength of the thick film dielectric layer is increased. Since the dielectric breakdown is associated with random defects in the dielectric layers, the probability of breakdown increases with increasing display area, and so layers with a higher nominal dielectric strength are used for larger area displays to counteract this tendency.
When a thick film dielectric electroluminescent display is constructed on glass substrates, the thick film dielectric layer is sintered at temperatures preferably close to the softening point of the glass, but not to the point where the glass deforms during display processing. Typically this temperature is near 700° C. Near this temperature, however, atomic species in the glass substrate, particularly those elements of Group IA of the Periodic Table, become mobile and may diffuse into the display structure, where they may cause functional degradation.
Constituent materials of the thick film dielectric layer, such as lead magnesium niobate (PMN) and lead magnesium titanate-zirconate (PMN-PT) are all high dielectric constant ferroelectric or para-electric materials by virtue of their perovskite crystal structure. PMN, however, may also form a pyrochlore crystal structure that is not ferroelectric and has a low relative dielectric constant. The formation of the pyrochlore phase has been reported to be suppressed by the addition of lead titanate to PMN in the thick film paste to form, upon sintering, PMN-PT, but the relative dielectric constant of this material will be too low for adequate display performance if the lead titanate concentration is too high. The tendency for pyrochlore formation may also be increased by the introduction of atomic species from the glass substrates.
It is apparent, therefore, that there is a need for a thick film dielectric electroluminescent display that obviates and mitigates some of the disadvantages previously discussed.